Microcomputer Circuits and Processes

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except that it is sent out bit after bit. This is an example of a serial format of data which you will meet in the next section. It took just four comparisons to convert the input signal into a 4-bit binary number. For a standard 8-bit conversion, only 8 comparisons are needed. That is much faster than the maximum of 255 comparisons for the linear ramp technique. The AD571 SAR-type converter comes with the D-to-A converter, SAR, comparator, and buffer on an 18-pin chip taking just 25 JlS for a conversion. For several thousand US dollars, you can even buy a SAR-type converter which will manage 10 million conversions per second. I!~r~ ~_~ __~~~ _!Q.~~R converter handles the transient sig~al. Gl Cl III ~ o > desired -'------ actual result measurement Time start conversion conversion complete Figure 4.31 Successive approximation conversion technique. Note how each D-to-A output increment is half of the previous increment. Here an 8-bit converter makes its eight comparisons, but only the first four are able to home in on the input voltage. It gets much closer to the desired value, but there is an error of another sort. Unlike the ramp converter which stops when it has reached the correct code, the 8-bit SAR must always execute its 8 comparisons. In this example, the converter fails to work properly after the fourth approximation. Its final result is a little too low. This problem is not major - in the example here, the converter was not given a fair chance. The engineer needs only to remember that this may happen, and to avoid it. In any case, the SAR technique is the most popular today, and for most laboratory uses represents the best tradeoff in terms of price versus speed. 64
The real winner for speed is the flash converter, which takes only nanoseconds to convert; the Plessey SP9754 is a 4-bit flash converter which can convert at 100 million conversions per second. That is fast enough to digitize video signals, and that is the area where you will find flash conversion at work. Perhaps you would expect such fantastic performance to require complex circuitry. It turns out that these are the simplest of all converters to understand. It is the actual manufacture which is difficult, and expensive. Figure 4.32 shows the line-up. It is literally a line of resistors of equal values which divide, here, a 10V reference into equal intervals. 10.0v------. 1 kll 8.0V-------
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General editor. Revised Nuffield Ad
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Longman Group Limited Longman House
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PROLOGUE This Reader explains how m
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transistor, through the logic circu
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tasks (large computing 'power'), wa
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800000 mark. Such high sales result
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CHAPTER 2 THE ELECTRONICS MICROCOMP
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enabled, as the truth table in figu
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D memory cell storing binary 0 stor
Page 20 and 21: address bus r----- AND gates RD .--
Page 22 and 23: If you get the chance, look up 7415
Page 24 and 25: Period 2 Period 3 The data bits are
Page 26 and 27: transfers the full 16 bits of addre
Page 28 and 29: INPUT AND OUTPUT Devices that input
Page 30 and 31: A glance at the timing diagram in f
Page 32 and 33: CHAPTER 3 RUNNING A SIMPLE PROGRAM
Page 34 and 35: adding is represented by 1111. Figu
Page 36 and 37: The number moved into register A im
Page 38 and 39: loaded into register A. So the CPU
Page 40 and 41: The FETCH cycles FETCH cycles, whic
Page 42 and 43: The EXECUTE cycles During EXECUTE c
Page 44 and 45: Second EXECUTE cycle MOV M--+Acc, c
Page 46 and 47: Third EXECUTE cycle ADI, clock peri
Page 48 and 49: That is the end of the program, exc
Page 50 and 51: CHAPTER 4 MEMORIES, INTERFACES, AND
Page 52 and 53: measurements made from a microproce
Page 54 and 55: (al (bl Figure 4.6 (a) Same as in f
Page 56 and 57: A typical bubble unit contains 300
Page 58 and 59: (al driver displays 0 1 1 0 1 0 0 1
Page 60 and 61: Where exactly is this common line c
Page 62 and 63: this R is in series with the next h
Page 64 and 65: That is the principle of the conver
Page 66 and 67: comparator digital-to-analogue data
Page 68 and 69: II CI l! ~L--~---- desired measurem
Page 72 and 73: These equally spaced voltages are f
Page 74 and 75: Figure 4.35 shows how the letter 'D
Page 76 and 77: A CONTROL AND DATA AQUISITION COMPU
Page 78 and 79: It does this by outputting a signal
Page 80 and 81: The second application, shown in fi
Page 82 and 83: INDEX This Index will direct you to
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Microcomputer Circuits and Processes

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